Both Uses

existential
in
This I Believe II
(Edited)

Each morning when I walk through the house on the way to make coffee, I turn on the music, hit the "shuffle" button, and it's Dance Time! I dance alone to whatever is playing. It's a form of existential aerobics, a moving meditation.
Chpt 24 *

existential = relating to what is important in life
For the first time in my life, I know how it feels to face an existential menace.
Chpt 50 *

existential = threatening existence

Definitions:

(1) (existential as in: existential threat)
relating to or dealing with existence -- especially with human existence
(2) (existential as in: existential philosophy)
Relating to existentialism — a philosophical movement where each person is free to determine their own meaning and purpose, not bound by convention, God, or authority

Existentialist belief is varied. All such beliefs stress the need for each individual to find their own way, but most stress that there is no rational way of finding it.

Atheistic existentialists often stress isolation of the individual in a hostile or indifferent universe. Some stress life without meaning. Others stress loss of a shared morality, uninhibited exercise of power, and/or hedonism.

Existentialism is often associated with philosophers and writers such as Kierkegaard, Nietzsche, Heidegger, Jaspers, Marcel, Sartre, Camus, Dostoevsky and Kafka.
(3) (meaning too rare to warrant focus)
Much more rarely, in logic, an "existential quantifier" denotes the existence of a member in a set to which something is applicable. It is contrasted to a universal quantifier which applies to all members of a set. For example: "What is the scope of the existential quantifier?"